1. Field of the Invention
The invention relates to a photographing apparatus which is suitable for use in a video camera or the like using an inner focus type lens.
2. Related Background Art
In recent years, in association with the realization of a small size and a light weight of a video integrated type camera, a volume and a weight of a lens section and an automatic focus adjusting section which occupy in the whole camera are rapidly being reduced. With respect to the latter automatic focus adjusting section, it is changed from the active type having a light projecting/receiving apparatus of infrared rays to the passive type to find out an in-focus point from a video signal derived through an image pickup signal without using such a light projecting/receiving apparatus.
On the other hand, with respect to the former lens section, there is frequently used what is called an inner focus type lens such that a focus adjusting function is also commonly provided for a lens to correct the movement of a focal plane by a variable magnification and, further, a front lens is fixed, thereby miniaturizing the lens section.
FIG. 1 shows an example of a construction of the above inner focus type lens. Reference numeral 101 denotes a fixed first lens group; 102 a second lens group (variable magnification lenses) to variably change a magnification; 103 an iris; 104 a fixed third lens group; 105 a fourth lens group (focus compensator lens) having both a function to correct the movement of a focal plane in association with the variable magnification and a focus adjusting function; and 106 an image pickup device.
FIG. 2 is a diagram of a characteristic curve showing the relation between the position (axis of abscissa) of the second lens group 102 which are moved to variably change the magnification and function as a zoom lens and the position (axis of ordinate) of the fourth lens group 105 which function as a focus lens in order to focus to the distance of each object to be photographed. The object distance is shown as a parameter. When the focal distance doesn't change, namely, when the second lens group 102 is stopped, the fourth lens group 105 moves in parallel with the axis of ordinate on the relevant focal distance, for instance, A--A' in FIG. 2, thereby performing the focus adjustment. During the zooming operation, namely, when the second lens group 102 is moving, a locus of the fourth lens group 105 is selected from FIG. 2 in accordance with each object distance and the fourth lens group 105 is driven and controlled in accordance with the selected locus in correspondence to a change in focal distance. Due to this, by providing both of the correcting function of the focal plane due to the variable magnification and the focus adjusting function for the fourth lens group 105, a video image without a blur can be also obtained even during the zooming operation.
FIG. 3 is a characteristic diagram for explaining an example of a drive control method of the fourth lens group 105, particularly, during the zooming operation. A method of setting the coordinates is similar to that in FIG. 2. However, in the control, the moving area of each of the zoom lens and the focus lens is divided into a plurality of zones as shown in FIG. 3 and the control is executed on a zone unit basis. Each arrow whose angle sequentially changes in FIG. 3 indicates a moving speed of the fourth lens group 105. In FIG. 3, a moving area (axis of abscissa) of the second lens group 102 is divided into equal sixteen areas. Each of the 16 equal areas is now called a "zoom zone".
The curve in FIG. 2 is divided every zoom zone. In this instance, the curve can be divided into the portions having almost the same gradient in each zoom zone. When the zoom speed is constant, so long as the gradients are equal, even if the object distances are different, the moving speeds of the fourth lens group 105 can be equalized. Therefore, as shown in FIG. 3, the axis of ordinate is divided into portions I, II, . . . having the same gradient every zoom zone and one representative speed is given, respectively.
With the above method, if the lens system is set into the in-focus state at the start of the zooming operation and the zooming operation is executed while detecting the positions of the zoom lens and focus lens, it is possible to always track the locus in FIG. 2 at the proper moving speed of the fourth lens group 105.
The moving speed information of the focus lens as described in FIG. 3 is obtained only when the moving speed of the zoom lens is fixed to a certain value (referred to as V.sub.zs). This is because the locus in FIG. 2 shows the position (axis of ordinate) of the focus lens to the position (axis of abscissa) of the zoom lens. To obtain the focus lens speed information to trace the locus, the moving speed of the zoom lens must be inevitably defined.
Therefore, for instance, assuming that an actuator of the zoom lens is constructed by a DC motor, the driving speed of DC motor fluctuates due to a difference of the position of the camera, a variation in torque of the actuator itself, environment, a reduced voltage of battery, or the like. If the zooming operation is executed while ignoring such a fluctuation, the focus lens speed to the zoom speed defined as mentioned above isn't fitted to the actual zoom speed, so that a blur occurs during the zooming operation.
Therefore, hitherto, as shown in, for example, JP-A-1-319717, there is proposed a method whereby the zoom speed during the zooming operation is always measured and a ratio between the result of the measurement and the standard moving speed V.sub.zs is calculated and multiplied to the moving speed information in FIG. 3.
FIG. 4 is a flowchart for explaining a measuring method of the zoom speed in the above conventional method. S1 denotes a step to indicate the start of the execution of a program. S.sub.2 denotes a step to execute a program to discriminate whether a zoom switch (not shown) has been depressed or not; S3 a step to execute a program to increase a value of a zoom speed measuring counter provided in order to measure the zoom speed by "1", S4 a step to execute a program to discriminate whether the zoom lens lies within a zoom zone boundary in FIG. 3 or not; S5 a step to execute a program to discriminate whether a vertical sync signal has been supplied or not; S6 a step to execute a program to substitute a value of a zoom speed storing memory 2 into a zoom speed storing memory 3; S7 a step to execute a program to substitute a value of a zoom speed storing memory 1 into the memory 2 in a manner similar to step S6; and S8 a step to execute a program to similarly substitute a value of the counter into the memory 1. Each of the zoom speed storing memories 1 to 3 denotes a memory to store a time which is required when the zoom lens is moved from one boundary of a certain zoom zone to another boundary by the number of vertical sync signals. S9 denotes a step to execute a program to clear the counter; S10 a step to execute a program to perform the normal AF (automatic focusing) operation when the operating mode is not the zooming mode; and S11 a step to indicate the end of execution of the program.
In the above flowchart, when it is confirmed that the execution of the program has been started in step S1 and the zoom switch has been depressed in step S2, the counter value is increased by "1" in step S3. When the zoom lens is located at a position out of the boundary of the zoom zone in step S4, the system waits for the arrival of the vertical sync signal in step S5. When the vertical sync signal is supplied, the counter value is again increased by "1" in step S3. When it is detected in step S4 that the zoom lens lies within the boundary of the zoom zone, the processes in step S6 and subsequent steps are executed. At this time point, a vertical synchronization period which is required to pass one zone has been stored in the counter and is used as zoom speed information. In steps S6 to S8, each time the zoom lens passes one zoom zone, the data is shifted so as to store the zoom speed information of the past three zones into the memories 3 to 1. When the latest zoom speed information is stored into the memory 1 in step S8, the counter is reset in step S9 and a series of zoom speed measuring processes are finished. Since no data exists in the memories 2 and 3 at the starting time point of the measurement, three data cannot be used for a predetermined period of time just after the start of the measurement.
With respect to the zoom speed information stored in the three memories as mentioned above, for instance, by calculating the mean value of them or by using the maximum value, the zoom speed information can be accurately used while eliminating the non-linearity or noise components of an encoder output.
In the above conventional apparatus, as shown in FIG. 3, a phenomenon such that the gradient of the locus suddenly increases for the axis of abscissa, particularly, at a position near the edge of telephoto (tele. T) occurs. Namely, this means that the moving speed of the focus lens during the zooming operation rapidly rises at a position near the tele. edge. It is known that such a tendency becomes typical as the focal distance increases (as it approaches to the right in the diagram). It is, therefore, necessary to prepare an actuator such that as a zoom magnification increases, enough rotational torque is obtained while realizing a desired speed at a position near the tele. edge. However, generally, in the actuator, as the user wants to obtain a high rotational speed and a high rotational torque, the size and noises increase and an electric power consumption also increases. Namely, when the zoom magnification increases, not only a size of actuator is enlarged but also an electric power consumption increases in association with the large zoom magnification. Thus, a size of battery or the like which is attached to an article is also enlarged, thereby producing a result opposite to the foregoing requirements of a small size and a light weight.
Therefore, as means of solving the above drawbacks, there has been proposed a method whereby a movement amount per unit time of the zoom lens is reduced in a region (namely, near the tele. edge) in which the moving speed of the focus lens is high.
When the operator wants to reduce the movement amount per unit time of the zoom lens at a position near the tele. edge as in the conventional apparatus, there is a large possibility in which both of the data before and after the measurement mixedly exist for a little while just after the change of the zoom speed and an erroneous result is derived. Accordingly, when the moving speed of the focus lens is determined while keeping such a small movement amount, a problem such that a blur during the zooming operation is rather promoted occurs.